Steps to reproduce:

Create a new 3D URP project
In the Hierarchy, right-click > UI Toolkit > Panel Renderer
Create new Source Asset (UI Document) and assign it to Panel Renderer
Open the UXML in UI Viewport
In the Style Sheets window click + and create new .USS file
In the Add new selector field, create any valid selector (optional)
In the Add new selector field, type .1 and press Enter
Save the file
Observe the Style Sheets window and Console

Actual results: The Style Sheets window accepts .1. After saving, the style sheet fails to import and all of its selectors disappear from the Style Sheets window, UI Builder, the UI Viewport and Play mode.

Expected results: A selector starting with a number is rejected

Reproducible with versions: 6000.6.3f1, 6000.7.0b3, 7000.0.0a1

Tested on (OS): macOS 26.5

Notes:

Error on save:

Assets/New USS.uss (line 10): error: Unsupported selector format. Selector names can not start with a digit. '.1 ' UnityEditor.AssetDatabase/AssetEditingScope:Dispose () Unity.UIToolkit.Editor.VisualElementEditingStage:Save ()

In UI Builder the same error is shown, but only the invalid selector is affected. It can be removed and the other selectors stay
Trying to re-add the same uss shows "Invalid USS File" error
The .uss file on disk still contains all selectors. Reimporting does not restore them. The only recovery is to open the .uss in a text editor and delete the .1 entry manually